Voltage imaging system using electro-optics

ABSTRACT

A two-dimensional image of the voltage distribution across a surface at a large plurality voltage test points of a panel under test is extracted by illuminating the surface with an input beam of optical energy through an electro-optic modulator wherein the modulator is disposed to allow longitudinal probing geometries such that a voltage on the surface of the panel under test causes a power modulation in the optical energy which can be observed through an area optical sensor (a camera) for use to directly produce a two-dimensional spatially-dependent power modulation image directly representative of the spatially corresponding voltage state on the surface of the panel under test. Surface crosstalk is minimized by placing the face of the modulator closer to the panel under test than the spacing of voltage sites in the panel under test. The device may operate in a passthrough mode or in a reflective mode.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 07/481,429 filed 2/15/90 for VOLTAGE IMAGING SYSTEM USINGELECTRO-OPTICS, now U.S. Pat. No. 4,983,911.

BACKGROUND OF THE INVENTION

This invention relates to electro-optic sensing of voltage, and moreparticularly, electro-optic sensing of voltage as a function of positionof a large plurality of voltage sources on a surface.

There is a growing need to be able to test and extract voltageinformation from a voltage-producing surface such as a printed circuitboard, an integrated circuit wafer (with or without a package) or aliquid crystal display panel in order to diagnose the integrity of thestructure. The present electronic test systems have a technology capableof testing circuit boards or panels (a panel under test or PUT) having anode density not exceeding approximately 100 nodes per square inch.

Certain applications, such as the testing of liquid crystal displaypanels are best practiced with non-contact sensing techniques such aselectro-optic techniques. However, these panels have conductive pixelssuch as transparent indium tin oxide (ITO), with a thin-film transistordeposited directly on the glass surface and are therefore, consideredvery fragile. Such panels also may have an additional insulating layercovering the deposited structure, rendering it impossible to placevoltage probes at selected positions on the panel. It is thereforeimpractical to impose contact testing on such panels. Nevertheless,current and projected production methods and strategies demand that eachpixel of such a panel be tested for its ability to change voltage stateas well an ability to measure voltage under various conditions relativeto their voltages. The current state of the art does not provide a testtechnique for such structures.

It is known to use electro-optic devices for serially testing selectednodes of a voltage producing device. Reference is made to U.S. Pat. Nos.4,875,006 and 4,862,075. The subject matter of those patents isincorporated herein and made a part hereof. Therein, the use of a singlelight beam is described to serially access individual sensor nodes usinga unique sensor/laser arrangement giving control over a beam of lightwhereby a Pockels Cell Modulator employs the electro-optic Pockelseffect to sense local electric fields produced by voltage on a surface.Such known devices require control of a beam by scanning technology suchas an acoustic-optic deflector or an x-y stage. Known systems are thuslimited to single beam, serial data extraction.

SUMMARY OF THE INVENTION

According to the invention, a two-dimensional image of the voltagedistribution across a surface at a large plurality of voltage testpoints of a panel under test is extracted by illuminating the surfacewith an input beam of optical energy through an electro-optic lightmodulating means such as an NCAP modulator or other liquid dispersedpolymer-based devices, wherein the light modulator is disposed to allowlongitudinal probing geometries such that a voltage on the surface ofthe panel under test causes a power modulation in the optical energywhich can be observed through an area optical sensor (such as a camera)for use to directly produce a two-dimensional spatially-dependent powermodulation image directly representative of the spatially correspondingvoltage state on the surface of the panel under test. Surface cross-talkis minimized by placing the face of the light modulator closer to thepanel under test than the spacing of voltage sites in the panel undertest. The device may operate in a pass-through mode or in a reflectivemode. In a pass-through mode, the image is sensed through a transparentpanel under test. In a reflective mode, light power is observed upontwo-pass reflection through the electro-optic light modulator. A cameraor other imaging sensor can be used as an instrument to detect thespatial image.

An apparatus operating in accordance with the method of the inventionincludes an optical energy source, such as a laser, an electro-opticsensor (light modulator) exhibiting an electro-optic effect, such as thelight scattering effect present in NCAP (nematic curvilinear alignedphase) or PDLC (polymer dispersed liquid crystal) films, when placed inclose proximity to the panel under test, and means for spatiallyobserving a spatially-modulated light beam.

The invention has particular application to noninvasive testing ofhigh-density integrated circuits and testing liquid crystal display(LCD) panels prior to connection with electrical interfaces.

The invention will be better understood by reference to the followingdetailed description in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a first embodiment of the invention.

FIG. 2 is a block diagram of a second embodiment of the invention.

FIG. 3 is a side cross-sectional view of an electro-optic crystaladjacent a panel under test.

FIG. 4 is a graph of an illustrative voltage spatial distribution inconnection with FIG. 3.

DESCRIPTION OF SPECIFIC EMBODIMENTS

FIGS. 1 and 2 illustrate two embodiments of a voltage imaging system 10or 10' for observing voltage at a large plurality of positions 12 on asurface 14 of a panel under test (PUT) 14 or 16. FIG. 3 illustrates adetail of a portion of the system 10. The PUT 14 is a panel transparentto optical energy at the wavelengths of interest and the PUT 16 is apanel which may be opaque to optical energy. The PUT 14 may be a siliconchip or the like and the PUT 16 may be a liquid crystal display (LCD)panel. In any case, the PUT 14 or 16 may be connected in some way to asource of power (not shown) in order to produce voltages at selectedsites on the surface 14 of the panel 14 or 16.

In order to probe the voltage, there is first a source 20 of opticalenergy, such as a xenon, sodium, or quartz halogen lamp, a pulsed orcontinuous laser or the like. The source optical energy is channeledinto a source beam 22 and processed to produce an optical input beam 24which may be expanded and collimated with a beam expander 28. For thispurpose, there may be provided a lens, mirror or fiber optic system 28to expand and collimate the source beam into the input beam 24. Thecollimated input beam preferably has a constant or at least known powerdensity cross-section.

The input beam is directed into an electro-optic modulator means 30 or31 of a specific type, structure and possibly atomic or molecular axisorientation. A suitable modulator 30 may be a modulator fabricated withan NCAP or PCLD film. This power modulator 30 utilizes the lightscattering properties of liquid crystal droplets encapsulated in apolymer matrix. The encapsulation structure produces a curvilinearalignment of the liquid crystal molecules, and this aligned phase can beoptically switched by a controlled electric field as desired. The deviceis therefore switching from a highly light scattering state to a highlytransmissive state. Biasing the device between the two states allows themodulator to exhibit a roughly linear light to voltage transferfunction. The electro-optic modulator means 30 has a first face 32 andan opposing second face 34 to allow longitudinal probing geometries, asexplained hereinafter. The first face 32 has a conductive coating 36which is transparent, such as indium tin oxide (ITO), and which iselectrically coupled to a voltage common 38, such as ground. The secondface 34 of modulator means 30 (FIG. 1) is transparent to permittransmission of the input beam through the PUT 16. The second face 34 ofmodulator means 31 (FIG. 2) has a highly-reflective nonconductivecoating 33, which produces a retro-reflection of the input beam 24. Thesecond face 34 is disposed to be adjacent an area 40 of the surface 14of the PUT 16 or 18. The modulator means 30 is oriented to intercept atleast a portion of the input beam 24 directed into the modulator throughthe first face 32 to impinge on the second face 34 at a position of themodulator means 30 or 31 closely adjacent the area 40 of the surface 14of the PUT 16 or 18. The voltages at positions 12 at or near the surface14 interact with the modulator means 30 to cause changes in the opticalpower transmission of the input beam 24 in alignment with positions 12of voltages. This is observable as a spatially-dependent change incollimated alignment of an output beam 42 with the input beam 24, thechanges being proportional to voltages at the positions 12 on thesurface 14.

FIG. 4 illustrates the interaction between a PUT 18 and a modulatormeans 31 of a type having a reflective coating 33 on the second face 34.The reflective coating may be a dielectric coating or stack ofdielectric material, and an air, solid or liquid gap is presumed toexist between the second face 34 and the surface 14. In operation, theelectric field (E-field) of each position 12 penetrates the modulatormeans 31 causing a change in the scattering or absorption properties ofthe light beam, preferably in collimated alignment with every point 12in direct and accurate proportion to the voltage at the point 12.Voltage V1 causes a positive change in power modulation. Voltage V2causes a negative change in power modulation. Voltage V3 causes apositive and extreme change in power modulation. The power modulation isa function of position and is directly proportional to the voltagedifferential between the surface 14 and the grounded conductive coating36 on the first face of the modulator 31. The beam exiting the modulator31, which in this example has made two passes through the modulator,therefore contains spatially modulated light power which carriesinformation regarding the voltage at each point 12 relative to thereference of coating 36.

The separation between the second face 34 and the surface 14 arecontrolled, preferably being as close as practical without causing sideeffects, such as shorts, thermal transfer or mechanical distortion dueto stress. The selection of the spacing is made to maximize the ratio ofsignal to noise, particularly noise attributable to cross-talk fromelectric fields produced by adjacent points of voltage. A working rule,applicable particularly to LCD panels wherein the source of voltage isan area defined as a pixel area (112 in FIG. 4), is to place the secondface 34 of the modulator relative to the surface 14 at less than thedistance between positions 12 and preferably no more than 30% of thediameter of the pixel area. The separation may be controlled by amechanical positioner, such as a movable table arrangement (not shown).

In order to extract that information, means are provided for detectingthe change in modulation in the image across the output beam 42 toanalyze the voltages. Referring to FIGS. 1 and 2, the detectors maycomprise means such as a sensitive camera 48 receiving light through forexample a focussing lens 46, which intercepts the spatially-dependentpower modulation for producing, as seen by the camera, an observable mapin two dimensions having features corresponding to voltage magnitude.Additionally, image processing can be used to enhance the ratio ofsignal to noise through manipulation in digitized format of the imagecaptured by the camera 48. (Digital image manipulation is a knowntechnique.)

In a single pass system, as shown in FIG. 1, imaging is in line. In amultiple pass system, as shown in FIG. 2, sensitivity may be enhanced bypassing the beam through the modulator twice. The output beam 42 isseparated from the collinear input beam 24 by means of a beam splitter50. As a still further alternative, the input beam and the output beamcan be separated by orienting the reflective surface 33 of the phasemodulator 31 so that it is not perpendicular to the incident radiation.The output beam 42 is thus separated upon reflection, and a beamsplitter is not needed.

The invention has now been described with reference to specificembodiments. Other embodiments will be apparent to those of ordinaryskill in the art. For example, multiquantum well electro-absorptionmodulators may be used. It is therefore not intended that this inventionbe limited, except as indicated in the appended claims.

What is claimed is:
 1. An apparatus for observing voltage at a largeplurality of positions on a surface of a panel under testcomprising:means for producing optical energy; means for directing saidoptical energy into an input beam of any polarization state; anelectro-optic modulator means, said electro-optic modulator means havinga first face and an opposing second face in an orientation to allowlongitudinal probing geometries, said first face having a conductivecoating electrically coupled to a voltage common and said second facebeing disposed to be adjacent an area of said surface of said panelunder test, said modulator means being oriented to intercept at least aportion of said input beam directed into said modulator means throughsaid first face to impinge on said second face adjacent said area ofsaid surface in order to cause in an output beam spatially-dependentmodulation along said input beam, said modulation being proportional tovoltages at positions on said surface; and means for detecting saidmodulation in an image across said output beam to analyze voltages onsaid surface.
 2. The apparatus according to claim 1 wherein saidelectro-optic modulator means is transmissive of said output beamthrough said surface and said surface is transmissive of said outputbeam such that optical energy passes only once through saidelectro-optic modulator means.
 3. The apparatus according to claim 1wherein said second face of said electro-optic modulator means has anoptically reflective coating such that said input beam and said outputbeam each pass through said electro-optic modulator means, furtherincluding means for separating said output beam from said input beam. 4.The apparatus according to claim 3 wherein said input beam and saidoutput beam are collinear, and said separating means is a beam splitter.5. The apparatus according to claim 1 wherein separations between saidpositions is greater than separation between said surface and saidsecond face in order to minimize cross-talk between electric fieldsproduced by voltages at said positions.
 6. The apparatus according toclaim 1 wherein said modulation is power modulation and wherein saiddetecting means comprises:means for producing an observable map havingfeatures corresponding to voltage magnitude.
 7. The apparatus accordingto claim 6 wherein said map producing means is a camera.
 8. Theapparatus according to claim 1 wherein said electro-optic modulationmeans is a crystal.
 9. The apparatus according to claim 1 wherein saidelectro-optic modulation means is a liquid crystal.
 10. The apparatusaccording to claim 9 wherein said second face of said electro-opticmodulator means has an optically reflective coating such that said inputbeam and said output beam each pass through said electro-optic modulatormeans, further including means for separating said output beam from saidinput beam and wherein said separating means is a beam splitter forsplitting optical energy.
 11. The apparatus according to claim 9 whereinseparations between said positions is greater than separation betweensaid surface and said second face in order to minimize cross-talkbetween electric fields produced by voltages at said positions.
 12. Theapparatus according to claim 1 wherein said electro-optic modulationmeans is a polymer-dispersed liquid crystal.
 13. The apparatus accordingto claim 1 wherein said electro-optic modulation means is a solidcrystal.
 14. An apparatus for simultaneously observing voltage at aplurality of positions on a surface of a panel under test comprising:alight source for producing optical energy; an electro-optic modulatormeans, said electro-optic modulator means having a first face and anopposing second face in an orientation to allow longitudinal probinggeometries, said first face having a conductive coating electricallycoupled to a voltage common and said second face being disposed to beadjacent an area of said surface of said panel under test, saidmodulator means being oriented to intercept at least a portion of saidinput beam directed into said modulator means through said first face toimpinge on said second face adjacent said area of said surface in orderto cause in an output beam spatially-dependent change in light poweralong said input beam, said change in light power being proportional tovoltages at positions on said surface; and means disposed to interceptsaid spatially-dependent power modulation for producing an observablemap having features corresponding to voltage magnitude to analyzevoltages on said surface.
 15. The apparatus according to claim 14wherein said electro-optic modulator means is transmissive of saidoutput beam through said surface and said surface is transmissive ofsaid output beam such that optical energy passes only once through saidelectro-optic modulator means.
 16. A method for observing voltage at alarge plurality of positions on a surface of a panel under testcomprising:directing an input beam of optical energy into anelectro-optic modulator means, said electro-optic modulator means havinga first face and an opposing second face in an orientation to allowlongitudinal probing geometries, said first face having a conductivecoating electrically coupled to a voltage common and said second facebeing disposed to be adjacent an area of said surface of said panelunder test, said electro-optic modulator means being oriented tointercept at least a portion of said input beam directed into saidmodulator means through said first face to said second face adjacentsaid area of said surface of said panel under test in order to cause inan output beam a spatially-dependent modulation along said input beam,said modulation being proportional to voltages at positions on saidsurface; and detecting said modulation in an image across said outputbeam to analyze voltages on said surface.
 17. The method according toclaim 16 wherein said modulation is power modulation and wherein saiddetecting step comprises:producing from said spatially-dependent powermodulation an observable map having features corresponding to voltagemagnitude.